US2391271A - Flux measurement - Google Patents

Description

A. RICH FLUX MEASUREMENT Filed pt. 1, 1945 Ln .mw R M 6 P 0 Z T Q His Attorney Patented Dec. 18,1945

FLUX MEASUREMENT Theodore A. Rich, Schenectady, N. Y., assignor to General Electric Company, a corporation or New York Application September 1, 1943, Serial No. 500,803

(Cl. 17195) r 7 Claims.

My invention relates to a method and apparatus for measuring unidirectional fluxes, and it is particularly beneficial for the measurement of direct current over a wide range and up to high values 7 of direct current flowing in large buses or cables by means of sensitive flux measuring apparatus.

The conventional method of measuring large values of direct current has been to connect a current shunt in the direct current circuit and measure the variation in voltage drop across such shunt with a low current instrument calibrated with the shunt and shunt leads in terms of line current. Where large direct current bus or cable structures are' used for carrying many thousands of amperes, such conventional apparatus becomes quite expensive and involves opening the direct current circuit to insert the shunt, since a special shunt having a negligible temperature coeiflcient of resistance must be used My invention may be used for measuring large direct currentsat' small expense and without the necessity of disturbing the direct current cables or bus. structure.

Fig. l, in the accompanying drawing, illustrates a schematic view of apparatus and circuit connections which may be used-in carrying out the invention. Fig. 2 illustratesra set of curves for due to such current flowing through resistor IS with the voltage drop produced therein by the current flowing in bus l0. Hence, so iar as the controlling action of instrument It is concerned, it tends to produce a current flow in the output circuit ,of tube I! through ammeter 2| which is very roughly proportional to the current flowing in bus in. However, without further refinements the apparatus thus far described could not be use in describing the operation of the apparatus.

Referring to Fig. 1, It) represents one cable orbus of a direct current circuit adapted to carry many thousands of amperes. The size and current carrying capacity of the bus and amount of current to be. measured are not limitations to the use of my invention. The bus I0 is shown asbeing. made up 01' several sections in accord-' ance with good practice where large currents are involvdpbut it will be assumed that-whatever current is flowing divides proportionally in the several sections in first explaining the action. Later it will be shown that this is not an essentialcondition of operation. Connected across a suitable short section of the bus I0 by leads and I2 and a variable resistance I3, is a sensitive detector of voltage I4 having a mirror on its moving element and which is used in combination with a light source l5 and a light sensitive cell It to control an amplifier tube I1. The tube i1 controls the current in a circuit includinga source of'supply i8, a'mutual inductance IS in parallel with a variable resistance 20, an ammeter 2|, and a tion of the variableresistance II. For a y give -setting oi the resistance l3 the action or instrument 'Il," phototube l6, and amplifier I1 is to 's'dontrol the current in the output circuit of the amplifier as to'balan'ce the voltage drop de nded upon to measure the current flow in bus ecause the bus section across which leads and i2 are connected is not a calibrated shunt and does not have current shunt characteristics and features necessary for accurate current measurement. The inexpensive controller thus far described does, however, serve to produce a useful comparison current which, after further regulation by means or an accurate flux measuring apparatus to be described, can be used ior accurate measurement purposes.

Also associated with current bus I0, is a flux pickup device consisting of coils 22 which may or may not be wound on magnetic strips 23 on opposite sides of the bu III. This flux pickup is mounted for rotation with a shaft 24 so that the axes of coils 22 may be turned from a position as to produce a flux linkage in a circuit 25, when the flux measuring coils are turned, proportional to the current flow in bus l0.

Included in circuit 25 is a sensitive fluxmeter 26 and coils of mutual inductances |9 and 21. It is v noted that the mutual inductance l9 has one of its coils 29 connected in the flux pickup circuit ,26 and the other coil 30 in the previously mentioned output circuit of tube I! in order that the current in-the flux pickup circuit mayrbe compared with that in the circuit of ammeter 2|. However, in order that such comparison may take place in approximately the same ratio asthe flux linkage in the flux pickup circuit 25 is induced therein by the turning of the flux pickup coils 22, the flux linkage in mutual inductance I9 due to current flow in the circuit of ammeter 2| is made to vary with the turning of the flux pickup coils 22. The coil 30 thereof which is in series with ammeter 2| could be mounted on shaft 24,

but it is more convenient to provide the resistance 20 in shunt to such coil and vary the resistance short circuit instead. Accordingly, the resistance 20 is arranged in shunt to the coil 30, and the variable resistance tap 3| of this resistance is mounted to turn with shaft 26 so that when coils 22 are turned to pick up flux, resistance 26 is inserted allowing current to flow in coil 36. Variation of resistance does not change the current flow in the circuit of meter 2|. When coils 22 are in a position to pick up zero flux, coil is short circuited and no current flows therein. The current flow in coil 30 induces in circuit 25 a voltage which is opposed to that induced inocircuit 25 by pickup coils 22, and hence, the resultant voltage induced in circuit 25 is the diflerence, if any, of such induced voltages, and such differential voltage is that which influences fluxmeter 26."

Instrument 26 has a mirror on its moving element and cooperates with a light source 32, a light beam divider 33, and a pair of photocell amplifiers 34 and 35 to control relays 36 and 31 which in turn control a reversible pilot motor 38 used to adjust the resistance |3 in a direction and extent as needed to regulate the current in the, output circuit of amplifier l1, and through coil 30 and ammeter 2| to the correct measurement value. For-example, the desired ratio of current flow in bus Ill and meter 2| may be 40,000 to 1 for correct measurement. If there are 40,000 amperes flowing in bus I 0 and 1.05 amperes flowing in meter 2|, a measurement operation will result. in the voltage of mutual inductance I9 being slightly in excess of that induced by flux picku 22, and instrument 26 will deflect to shine light on photocell amplifier 34 which will operate relay 36 and cause motor 38 to rotate in a direction to increase the resistance I3. This will reduce the deflection of instrument M, which will decrease the positive bias of amplifier tube I1 and decrease the current in ammeter 2| and coil 30 until instrument 26 returns to a zero deflection when the regulation will stop, at which time meter 2| will read correctly. On the other hand, if the current in the measurement circuit 01 meter 2| is too low, the voltage of pickup coil 22 will predominate and cause instrument 26 to deflect in the other direction, causing light to'shine on photocell amplifier 35, thereby energizing relay 3'1 -and rotating motor 38 in a direction to decrease scribed in United States Patent No. 1,897,850 to La Pierre, February 14, 1933.

Following a correcting calibration operation as above described, by the rotation of coil 22 and associated rheostat arm 3|, and switch 42, the apparatus will be left in the condition represented, with a deflection of instrument l4 and an adjustment of resistance I3 which will cause a current to flow in ammeter 2| correctly proportional to that in bus I6. Now, so long as there is no change in temperature in bus In or other conditions influencing the calibration, ammeter 2| will continue to indicate correctly the current flowing in bus l0 and will follow the variations in such current. The flux measurement system serves as a monitor. The current in ammeter 2| normally follows the bus current with only minor errors, so that a continuous measurement indication is obtained, and the rotation of the flux pickup coil is ior'the occasional correction of these minor errors as needed by recalibration of the shunt I, l2, l3, and I4, and the regulating apparatus described.

When the pickup coil is rotated to take a measurement, it may happen that the rate of flux change in its coil with angular deflection and the rate of change of flux in mutual inductance 29 due to the shorting effect of resistance 20 may be quite different over the flux coupling range of movement even though the final resultant change is the same, because it will be difficult to match this characteristic of these two devices. This is illustrated in the curves of Fig. 2 where the ordinates represent integrated flux linkage of circuit 25 and the abscissa the angular movement of the search coil. Let curve S, Fig. 2, represent the integrated number of lines of flux linked by coils ,22 due to a given current flow in bus III as the coils are turned through an angle of degrees to the position shown in Fig. 1. At 0 angle the flux linkage is zero and at 90 degrees the fiux linkage rises to a maximum value lines with circuit 25. The linkage of mutual inductance I9 is made to oppose that of curve S and thus the fiux linkage of mutual inductance I9 is plotted negative and may be represented by curve M, it being understood that both curves are plotted to the same fiux unit scale. The difference between the plus and minus flux linkage is represented by the dotted dash curve R. In the example given it is seen that the total resultant final flux linkage at the end of the 90- degree swing has a positive value X, indicating that the current in the measurement circuit of meter 2| is too low and should be adjusted accordingly, by an amount to bring the value X to zero, at which time meter 2| will correctly indicate the current flow in cable I.

If search coils 22 were the only flux linkage coils in the circuit 25, fiuxmeter 26 would attempt to deflect in one (positive) direction according to the curve S. That is, at 45 degrees swing, meter 26 would attempt to deflect to an amount ds, etc. If mutual inductance l9 was used alone with the same value of current in its primary, the instrument 26 would attempt to deflect by an amount dm in the negative or opposite direction, at 45 degrees swing. Owing to the bucking relation of the fluxes S and M, the meter 26 attempts to deflect according to curve R and, if capable of doing so, would follow the curv R and have a final deflection X in the positive direction. We are interested in only the deflection X for measurement and regulating purposes and it is desirable for accurate results that the fluxmeter 26 be of such sensitivity as to produce a good regulating deflection for th value X and other smaller final resultant flux measurements. However, if we used a fiuxmeter having the high sensitivity necessary for good measurement of fiux values corresponding to X, and took no further precautions, such meter would be banged ofi scale, probably damaged, and in any event, the resultant flux integration measurement destroyed by reason of the large resultant flux linkage dr, for example. To prevent such results and still enable me to use a fiuxmeter at 26 with the desired high sensitivity, 1 prosubstantially completed that shown in Fig. 2. The relays can ance 21 having winding :9 in circuit 25 and windings and 4| in the output circuits of photoinstrument 26, which causes current to flow in coils 44 and 4|. Thus, when R causes instrument :6 to deflect to direct light on photocell amplifler 34, the output current flows in coil 40 and induces a voltage in circuit 25 which bucks verslng the connections of coils 22 to circuit and short circuiting coils 40 and 4| at the other end of the movement of the flux linkage operation, the measurin and regulating operation could be made just as well by moving coils 22 from flux linking to nonflux linking position, as

in the reverse direction, because either operation theresultant R and, as a result, the deflection of instrument 26 is held down or the sensitivity of the flux measuring system as a whole but not the fluxmeter itself is, in effect, greatly reduced until close to the end of the flux linking operation, at which time the secondary coils 40 and 4| are short circuited by a switch arm 42 and switches 43 and 44 operated from shaft 24. The deflection of instrument '26, instead of attempting to go to dr, Fig. 2, is then only a small fraction thereof such as curve E, Fig. 2, and it is only when the flux linking operation has been instrument 26 is energized by the full difference between flux linkages S and M or K, Fig. 2. The device 39 serves to store the excess differential flux linkage until near the end of the calibrating operation.

It is also to be noted that the coils 46 and 4| are in series with the coils of relays 36 and 31 until the coils 40 and 4| are Until the coils 40 and 4| are short circuited, the deflection is small. However, when the coils 40 and 4| are short circuited, the resultant change of current produces a change in flux linkage in circuit and now instrument 26 gets a deflection which is the equivalent of the change X then be operated by the output current of the photoelectric amplifiers 34 or 35, and this will cause operation of the pilot motor to increase or decrease the output-of H to correct the ratio of currents. Once a relay is energized, it stays energized until the short circuited.

produces a flux linkage change in pickup coils 22 proportional to the current flow in bus l0. When the flux pickup coils are in the neutral position parallel to bus l6, resistance 20 is short circuited and switches 43 and 44 are open. A current is flowing inv instrument 2| and switch arm 3| roughly proportional to that in bus that required for correct calibration. Instrument 26 is in a neutral position since, if not in such position, it will automatically be moved to such position by reason of current flow in one or the other of coils 46 or 4|. 'Shaft 24 is now turned through an angle of 90 degrees and.

stopped in. the maximum flux linkage position with respect to bus I6 by suitable stops which may be represented in Fig. 1 by the stationary switch contacts 43 and 44. The rate at which such movement takes placeis immaterial; If the flux linkage curves S and M do not coincide,

coils Or 4| will be energized to hold the re- 1 sultant flux linkage in circuit 25 and the deflection of fluxmeter 26 down to a low value until such coils are short circuited at the end of the operation, at which time if K, Fig. 2, is not zero,

instrument 26 will deflect one way or the other depending upon whether X, Fig. 2, is positive or negative, the corresponding relay 34 or 35 will be energized, and motor 38 will run in one direction or the other to decrease or increase the resistance at l3. This action will continue perhaps for several cycles until the current in coil 30 mutual inductance has reduced or increased its flux linkage to reduce the net change in flux deflection instrument 26 has been returned to nearly its zero position. Due to the return spring on 26 this will occur in a few seconds in any event, in which case complete correction may not be obtained in a single operation. The above cycle is reversed to bring coils 22 and 30 to their original position with the pilot motor deenergized. A repetition of the correcting action described above will bring the ratio even closer to the required value. The changes in ratio are primarly due to changes in bus resistance and since these changes are slow this checking comparison need not be rapid. For example, a correction cycle and return can be made in 13 seconds, and good accuracy can be obtained by say four cycles in succession followed by a wait of 10 minutes.

1 The operation of the apparatus to make a current measurement is as follows: The apparatus is connected as described. If the flux pickupv coils are not in the neutral position parallel to bus ll, they are moved to that position. Such movement of course produces flux linkage changes but instrument 26 is protected, as above described, and no regulation'of resistor I3 results. In this connection it may be noted that by relinkage of circuit 25 to zero, so that fluxmeter 26 has no net deflection and the regulation will cease. The current flowingin bus I0 is then correctly indicated on ammeter 2|.

The principle of the use of a protective or compensating mutual inductance, such as represented at 39 in Fig. 1, and which serves to prevent damage to the flux-meter without reducing its sensitivity, is capable of, general application and is very useful. For example, in Fig. 1, supposeit was simply desired to measure the flux linkage of coils 22 with great accuracy, and that the mutual inductance'at IS, the switch 42, and the relays 36 and 31 were omitted. The mutual inductance at 39 would serve to protect the sensitive fluxmeter as before, and at the, end of a flux measurement linkage operation, the amount of flux linkage contributed by coil 39 could be accurately determined from its mutual inductance ratio and a measurement of the current in the direct current circuit of its primary coil 40 or 4|.

The deflection of the fluxmeter would then correspond to the flux linkage of coils 22 less the known flux linkage produced at l9. In this way a fluxmeter having the sensitivity capable of being built into a null type instrument may be used to measure fluxes covering a wide range with very high accuracy.

In accordance with the provisions of the patent statutes, I have described the principle of operation of my invention together with the apparatus which I now consider to represent the best embodiment thereof, but I desire to have it understood that the apparatus shown is only illustrative Ill, although it may be several per cent higher or lower than V other means.

and that invention may be carried out by tion. and means for reducing any compensating flux to zero at the end or the measurement flux What I claim as new and desire to secure by Letters Patent the United States is:

i. A direct current measuring system in combination with the conductor in which the current is to be measured, a direct current metering circult the current in which is regulated in response linking operation.

'4, Inaflux measuringsystem, aflux measuring circuit including a sensitive fluxmeter, means for linking said circuit with diflerent fluxes the algebraic sumpi' whichiis to .be measured but under conditions such vthat during the flux linksite directions such that thefluxmeter responds to the diflerence in such flux linkages, and means.

controlled by said fluxmeter for calibrating the regulation of the metering circuit as needed to cause the current flow therein to be a predetermined fraction of the current flow insaid conductor.

2. A direct current measuring system comprising in combination with a conductor carrying the current to be measured, a circuit connected across a section of said conductor so as to obtain a voltage approximately proportional to the current flow in said conductor, said circuit including a variable resistance and a sensitive voltage detecting instrument, a metering circuit in which said variable resistor is also included, means in the metering circuit controlled in response to said voltage detecting instrument for regulating the current flow in said metering circuit tending to maintain said current proportional to the voltage ing operation the circuit is likely to be sub- Jected to a resultant flux linkage in excess of the flnal nus linkage 'to be measured and such that the fluxmeter is likely to be damaged, means for protecting the fluxmeter during such flux linkage measuring operation comprising means for linking the flux measuring circuit with a compensating flux which is proportional to the fluxmeter deflection and which is in a direction to reduce the fluxmeter deflection, and means for eliminating any compensating flux linkage at the end oi 'the measurement flux linkage p ration whereby the flnal deflection of the fluxmeter corresponds to the algebraic sum of the measureured, a mutual inductance having a coil in said drop across said conductor section, a flux linkage measuring circuit having a coil linked with the flux fleld of said conductor, a mutual inductance having one coil in said metering circuit and the other coil in said flux linkage measuring circuit,

a sensitive flux measuring instrument connected in the flux linkage circuit for measuring difl'erences in the flux linkage by said conductor and mutual inductance, and means controlled in response to the differential measurement oi said flux measuring instrument for adjusting said variable resistance in a direction to maintain the current! in, said metering circuit proportional to th flux field of said conductor.

3. In a differential flux measuring system comprising a flux measuring circuit including a sensitive fluxmeter, means for causing said circuit to be difierentially linked with two fluxes to flux measuring circuit and another coil in a .direct current circuit, and means responsive to the deflection of said fluxmeter for controlling the current in said direct current circuit in proportion to such deflection, said mutual inductance producing a flux linkage in the flux measuring circuit in a direction which tends tore-'- duce the deflection of said fluxmeter and serves to protect the fluxmeter from excessive deflections without reducing its measurement purposes.

6. In a diiierential flux measuring system, a flux measuring circuit including a. fluxmeter, and a coil which is movable from nonlinking to linking position with respect to a, measurement flux,

. means other than through said coil for introducing another flux linkage into said flux measuring circuit which is in opposition to the measurement flux linkage, means for controlling the magnitude of such opposition flux linkage, and means for simultaneously moving said coil from nonfiux linkage to flux linkage position and operating said controlling means to increase the opposing flux linkage.

7. In adiflerential fluir measuring system, a flux measuring circuit including a fluxmeter, a movable coil in said circuit adapted to be moved to link with a measurement flux, a. mutual inductance having a secondary winding in said flux measuring circuit and a, primary winding in a direct current circuit for producing a flux linkage in the flux measuring circuit which is opposed to the measurement flux, means for controlling the current flow in said primary winding, and means for simultaneously moving the V movable coil from nonflux linkage to flux linkage position and operating said current controlling means to increase the current flow in said primary THEODORE A. RICH.

sensitivity for flux-

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